A compound Z with molecular formula $C_3H_9N$ reacts with $C_6H_5SO_2Cl$ to give a solid, insoluble in alkali. Identify Z.

> [!TLDR] > With the Hinsberg reagent $\mathrm{C_6H_5SO_2Cl}$, a **secondary** amine gives a sulfonamide with no N–H, which is a solid **insoluble in alkali**. The only secondary amine of formula $\mathrm{C_3H_9N}$ is **N-methylethanamine (ethylmethylamine)**. ## Reasoning $\mathrm{C_3H_9N}$ (fully saturated) has four amine isomers: | Structure | Type | |---|---| | $\mathrm{CH_3CH_2CH_2NH_2}$ (propan-1-amine) | primary | | $\mathrm{(CH_3)_2CHNH_2}$ (propan-2-amine) | primary | | $\mathrm{CH_3CH_2NHCH_3}$ (N-methylethanamine) | secondary | | $\mathrm{(CH_3)_3N}$ (trimethylamine) | tertiary | In the Hinsberg test: - a **primary** amine gives a sulfonamide retaining one N–H, which is acidic and **dissolves** in alkali; - a **secondary** amine gives a sulfonamide with **no N–H**, a solid that is **insoluble** in alkali; - a **tertiary** amine has no N–H and does not form a sulfonamide. …

A compound Z with molecular formula C₃H₉N reacts with C₆H₅SO₂Cl to give a solid, insoluble in alkali. Identify

NCERT Exemplar · Q43

C_3H_9N

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Concept understanding — Oxidation Reactions

Alcohol Oxidation: The Intuition First

Imagine you have a molecule of ethanol — the alcohol in your hand sanitizer or a drink. It has a carbon atom bonded to an –OH group. Now picture that –OH group as a "handle" that can be transformed. Oxidation, in organic chemistry, doesn't always mean adding oxygen — it often means removing hydrogen from a carbon that already has a bond to oxygen. For alcohols, oxidation is like "stripping away" hydrogen atoms from the carbon that holds the –OH, turning the alcohol into a more oxidized functional group.

Think of it this way: a primary alcohol (R–CH₂–OH) has two hydrogens on the carbon with the –OH. If you remove one hydrogen and the hydrogen from the –OH, you get an aldehyde (R–CHO). Remove both hydrogens (and the –OH hydrogen), and you get a carboxylic acid (R–COOH). A secondary alcohol (R–CHOH–R') has only one hydrogen on that carbon — remove it, and you get a ketone (R–CO–R'). A tertiary alcohol has no hydrogen on that carbon — so it cannot be oxidized without breaking the carbon skeleton.

That's the core intuition: oxidation of an alcohol is about removing hydrogens from the carbon bearing the –OH group. The more hydrogens you can remove, the more oxidized the product.

The Precise Statement

Alcohol oxidation is the process in which an alcohol loses hydrogen atoms (dehydrogenation) from the carbon bonded to the -OH group, increasing the number of C-O bonds (or decreasing C-H bonds). The outcome depends on the class of the alcohol:

Alcohol Class	Structure	Product after oxidation	Reagent example
Primary (1°)	R–CH₂–OH	Aldehyde (R–CHO) then Carboxylic acid (R–COOH)	PCC (stops at aldehyde); K₂Cr₂O₇/H⁺ (goes to acid)
Secondary (2°)	R–CHOH–R'	Ketone (R–CO–R')	K₂Cr₂O₇/H⁺, CrO₃, etc.
Tertiary (3°)	R₃C–OH	No reaction (under normal conditions)	—

Watch out

A common mistake: students think "oxidation" always adds oxygen. For alcohols, it's removal of hydrogen from the carbon with the –OH. The oxygen from the –OH stays — it's the hydrogens that leave.

Why Does Tertiary Alcohol Not Oxidize?

Look at the carbon with the –OH in a tertiary alcohol: it has three carbon groups attached and no hydrogen. To form a C=O bond, you'd need to remove a hydrogen from that carbon — but there is none. The only way to oxidize a tertiary alcohol is to break a C–C bond (strong and difficult), which is not typical oxidation. So in standard organic chemistry, tertiary alcohols are inert to mild oxidizing agents.

A Real-World Analogy

Think of the alcohol carbon as a "parking spot" with a certain number of hydrogen "cars." Primary alcohol has two cars parked. Oxidation is like towing away one car (→ aldehyde) or both cars (→ carboxylic acid). Secondary alcohol has one car — tow it away, and you get a ketone. Tertiary alcohol has zero cars — nothing to tow, so no reaction.

Key Reagents to Remember (for exams)

PCC (pyridinium chlorochromate): oxidizes 1° alcohols to aldehydes only — stops there.
K₂Cr₂O₇ / H₂SO₄ (Jones reagent): oxidizes 1° alcohols all the way to carboxylic acids; 2° alcohols to ketones.
KMnO₄: similar to dichromate, but stronger — can over-oxidize. …

Why this formula?

Oxidation Reactions: Why the Key Principles Hold

Oxidation reactions are fundamental to chemistry, and understanding why they work the way they do is essential for mastering Indian board exams (Class 11, 12, JEE, NEET). Let's break down the core ideas from first principles.

1. The Core Definition: What Does "Oxidation" Really Mean?

Historically, oxidation meant "adding oxygen." But that's too narrow. The modern, exam-correct definition is:

Oxidation is the loss of electrons by a species.

This is the electronic concept (given by the ionic theory). The why behind this definition comes from the behavior of atoms during chemical reactions.

Why do atoms lose electrons?

Atoms seek stability. They achieve this by having a full outer electron shell (octet, duplet, or pseudo-inert gas configuration).

Metals (like Na, Mg, Fe) have few valence electrons (1, 2, or 3). It's energetically easier for them to lose these electrons than to gain 5, 6, or 7.
Non-metals (like O, Cl, F) have many valence electrons (5, 6, or 7). It's energetically easier for them to gain electrons.

So, when a metal reacts with a non-metal, the metal loses electrons (gets oxidized), and the non-metal gains electrons (gets reduced).

Example:

2\text{Na} + \text{Cl}_2 \rightarrow 2\text{NaCl}

Na loses 1 electron: $\text{Na} \rightarrow \text{Na}^+ + e^-$ (Oxidation)
Cl gains 1 electron: $\text{Cl}_2 + 2e^- \rightarrow 2\text{Cl}^-$ (Reduction)

Key takeaway: Oxidation and reduction always happen together (Redox reactions). You cannot have one without the other.

2. The Key Formula(e): Oxidation Number Rules

The oxidation number (O.N.) is a bookkeeping tool. It's not a real charge (except in ionic compounds), but it helps track electron flow.

Why do we assign oxidation numbers?

Because in covalent compounds (like $\text{CH}_4$ or $\text{H}_2\text{O}$ ), electrons are shared, not transferred. We need a way to pretend they are transferred to see which atom "owns" the electrons more.

The Rules (and why they exist)

Rule	Statement	Why this rule?
1	O.N. of an element in its free state = 0	No electron transfer has occurred.
2	O.N. of a monatomic ion = its charge	The atom has actually lost/gained that many electrons.
3	O.N. of H = +1 (except in metal hydrides where it's -1)	H is less electronegative than O, F, Cl, but more electronegative than metals.
4	O.N. of O = -2 (except in peroxides where it's -1, superoxides -1/2, and with F where it's +2)	O is highly electronegative (3.44 on Pauling scale). It "pulls" electrons toward itself.
5	Sum of O.N. in a neutral compound = 0	The compound has no net charge.
6	Sum of O.N. in a polyatomic ion = charge of the ion	The ion's overall charge must be accounted for.

The Derivation of a Key Formula: Finding O.N. of an Unknown Element

Suppose you need to find the O.N. of S in $\text{H}_2\text{SO}_4$ .

Step 1: Write known O.N.s:

H: +1 (rule 3)
O: -2 (rule 4)
S: let it be $x$ (unknown)

Step 2: Apply rule 5 (neutral compound sum = 0):

2(+1) + x + 4(-2) = 0

Step 3: Solve:

2 + x - 8 = 0

x - 6 = 0

x = +6

Why this works: The oxidation number is a mathematical consequence of the electronegativity hierarchy. Oxygen is more electronegative than sulfur, so it "takes" the electrons. Hydrogen is less electronegative than sulfur, so it "gives" electrons to sulfur. The net result is that sulfur appears to have lost 6 electrons.

3. The Key Formula(e): Balancing Redox Equations

Two methods are exam-critical: Oxidation Number Method and Ion-Electron Method (Half-Reaction Method).

Why do we need these methods?

Because in a redox reaction, the total number of electrons lost (oxidation) must equal the total number of electrons gained (reduction). This is the Law of Conservation of Charge.

The Ion-Electron Method (for acidic medium) — Step-by-step why

Example: Balance $\text{MnO}_4^- + \text{Fe}^{2+} \rightarrow \text{Mn}^{2+} + \text{Fe}^{3+}$ (acidic)

Step 1: Write half-reactions.

Oxidation: $\text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + e^-$

Why? Fe loses 1 electron (O.N. goes from +2 to +3).
Reduction: $\text{MnO}_4^- \rightarrow \text{Mn}^{2+}$

Why? Mn gains electrons (O.N. goes from +7 to +2).

Step 2: Balance atoms other than H and O.

Mn is already balanced (1 on each side).

Step 3: Balance O by adding $\text{H}_2\text{O}$ .

Left: 4 O atoms. Right: 0 O atoms.
Add 4 $\text{H}_2\text{O}$ to the right:

\text{MnO}_4^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O}

Step 4: Balance H by adding $\text{H}^+$ (because acidic medium).

Right: 8 H atoms (from 4 $\text{H}_2\text{O}$ ). Left: 0 H atoms.
Add 8 $\text{H}^+$ to the left:

8\text{H}^+ + \text{MnO}_4^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O}

Step 5: Balance charge by adding electrons.

Left: $8(+1) + (-1) = +7$ charge.
Right: $+2$ charge.
To make left = right, add 5 electrons to the left:

8\text{H}^+ + \text{MnO}_4^- + 5e^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O}

Step 6: Multiply half-reactions to equalize electrons.

Oxidation: $\text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + e^-$ (×5)
Reduction: $8\text{H}^+ + \text{MnO}_4^- + 5e^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O}$ (×1)

Step 7: Add them:

5\text{Fe}^{2+} + 8\text{H}^+ + \text{MnO}_4^- \rightarrow 5\text{Fe}^{3+} + \text{Mn}^{2+} + 4\text{H}_2\text{O}

Why this works: Every step is driven by conservation laws:

Mass balance: Same number of each atom on both sides.
Charge balance: Net charge on left = net charge on right.
Electron balance: Electrons lost = electrons gained.

4. The Key Formula(e): Electrochemical Series and Cell Potential

For a galvanic cell (voltaic cell), the cell potential $E^\circ_{\text{cell}}$ is:

E^\circ_{\text{cell}} = E^\circ_{\text{cathode}} - E^\circ_{\text{anode}}

Why this formula? …

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With the Hinsberg reagent $\mathrm{C_6H_5SO_2Cl}$ , a secondary amine gives a sulfonamide with no N–H, which is a solid insoluble in alkali. The only secondary amine of formula $\mathrm{C_3H_9N}$ is N-methylethanamine (ethylmethylamine).

Reasoning

$\mathrm{C_3H_9N}$ (fully saturated) has four amine isomers:

Structure	Type
$\mathrm{CH_3CH_2CH_2NH_2}$ (propan-1-amine)	primary
$\mathrm{(CH_3)_2CHNH_2}$ (propan-2-amine)	primary
$\mathrm{CH_3CH_2NHCH_3}$ (N-methylethanamine)	secondary
$\mathrm{(CH_3)_3N}$ (trimethylamine)	tertiary

In the Hinsberg test:

a primary amine gives a sulfonamide retaining one N–H, which is acidic and dissolves in alkali;
a secondary amine gives a sulfonamide with no N–H, a solid that is insoluble in alkali;
a tertiary amine has no N–H and does not form a sulfonamide. …

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The Concept: Hinsberg Test for Amines

The Hinsberg test uses benzenesulfonyl chloride ( $C_6H_5SO_2Cl$ ) to distinguish between primary, secondary, and tertiary amines.

Primary amine (1°) → forms a sulfonamide that is soluble in alkali (due to acidic N–H).
Secondary amine (2°) → forms a sulfonamide that is insoluble in alkali (no acidic H on N).
Tertiary amine (3°) → no reaction (no H on N to replace).

Here, the product is insoluble in alkali, so Z must be a secondary amine.

Step 1: Identify possible isomers of $C_3H_9N$

The molecular formula $C_3H_9N$ corresponds to saturated amines (no double bonds or rings). Possible isomers:

Type	Structure	Name
1° amine	$CH_3CH_2CH_2NH_2$	Propylamine
1° amine	$(CH_3)_2CHNH_2$	Isopropylamine
2° amine	$CH_3CH_2NHCH_3$	Ethylmethylamine
2° amine	$(CH_3)_2NH$ ? No — that's $C_2H_7N$	Not possible here
3° amine	$(CH_3)_3N$	Trimethylamine

Note that $(CH_3)_3N$ (trimethylamine) is $C_3H_9N$ — 3 carbons, 9 hydrogens, 1 nitrogen — so it is a possible tertiary amine.

So the secondary amine among these is only ethylmethylamine ( $CH_3CH_2NHCH_3$ ).

Step 2: Apply Hinsberg test logic

If Z were a primary amine → product soluble in alkali → contradiction.
If Z were a tertiary amine → no reaction → no solid formed → contradiction.
Therefore, Z must be a secondary amine.

Answer: Z is ethylmethylamine ( $CH_3CH_2NHCH_3$ ).

Common Mistakes Students Make

✗ Mistake 1: Forgetting that tertiary amines give no solid

Why it happens: Students memorize "insoluble in alkali = secondary" but forget that tertiary amines don't react at all.
How to avoid: Always check: if the question says "gives a solid", tertiary amine is ruled out immediately.

✗ Mistake 2: Confusing solubility direction

Why it happens: Mixing up which amine type gives soluble vs insoluble product.
How to avoid: Remember: primary = soluble (because the N–H is acidic enough to form a salt with KOH/NaOH). Secondary = no acidic H → insoluble.

✗ Mistake 3: Listing wrong isomers …

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Q.A compound Z with molecular formula $C_3H_9N$ reacts with $C_6H_5SO_2Cl$ to give a solid, insoluble in alkali. Identify Z.

Concept understanding — Oxidation Reactions

Alcohol Oxidation: The Intuition First

The Precise Statement

Why Does Tertiary Alcohol Not Oxidize?

A Real-World Analogy

Key Reagents to Remember (for exams)

Why this formula?

Oxidation Reactions: Why the Key Principles Hold

1. The Core Definition: What Does "Oxidation" Really Mean?

Why do atoms lose electrons?

2. The Key Formula(e): Oxidation Number Rules

Why do we assign oxidation numbers?

The Rules (and why they exist)

The Derivation of a Key Formula: Finding O.N. of an Unknown Element

3. The Key Formula(e): Balancing Redox Equations

Why do we need these methods?

The Ion-Electron Method (for acidic medium) — Step-by-step why

4. The Key Formula(e): Electrochemical Series and Cell Potential

Why this formula? …

Reasoning

Concept: Hinsberg Test for Amines

Method: Hinsberg Test Analysis

The Concept: Hinsberg Test for Amines

Step 1: Identify possible isomers of $C_3H_9N$

Step 2: Apply Hinsberg test logic

Common Mistakes Students Make

✗ Mistake 1: Forgetting that tertiary amines give no solid

✗ Mistake 2: Confusing solubility direction

✗ Mistake 3: Listing wrong isomers …

Q.A compound Z with molecular formula C3H9NC_3H_9NC3​H9​N reacts with C6H5SO2ClC_6H_5SO_2ClC6​H5​SO2​Cl to give a solid, insoluble in alkali. Identify Z.

Reasoning

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Q.A compound Z with molecular formula $C_3H_9N$ reacts with $C_6H_5SO_2Cl$ to give a solid, insoluble in alkali. Identify Z.

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